As electronic circuits are increasingly made smaller and more integrated, an increasing variety of portable products are made available. All of these portable electronic device, such as portable computers and cellular radio-telephones, for example, are powered by a battery or battery pack. Typically it is less costly to operate the device if the device is powered by a secondary or rechargeable battery or battery pack. This is because, while a secondary battery may only store half as much energy as a primary or single-use battery, it can be recharged hundreds of times, significantly reducing the operation cost compared to primary batteries.
At the same time, increasingly sophisticated devices are designed that optimize the usage of the battery. Circuits such as the so called battery fuel gauge are more common place. There is also a heightened awareness of safety, and safety circuitry is often included in the battery pack. This is especially true of battery packs using lithium ion cells, where strict voltage limits must be observed to both ensure performance and reduce safety hazards. Furthermore, it is becoming more commonplace to place a memory device inside the battery pack to store certain parameters, such as cycle count information, cell chemistry information, and so on.
As the amount and sophistication of circuitry in battery pack designs increase, there is a growing concern regarding the current drain of this circuitry. Current used by the battery pack circuitry reduces the amount of energy available to the host device for which the battery is designed to power. One facet of battery pack manufacture that is of particular concern with regard to battery pack circuit current drain is the period of time between when the battery pack is assembled and the time it is first charged by an end customer. During this time, which may be on the order of months, the circuitry in the battery pack will typically be consuming energy, resulting in the battery charge level continuously decreasing. If the charge level decreases too far, a condition referred to as over-discharged, the life cycle capability of the battery pack will likely be shortened. At an extreme, the battery pack may be significantly damaged, and unable to provide the full charge capacity it would otherwise be capable of delivering. This is particularly true in the case of a lithium ion battery pack.
In addressing this problem there are two main strategies. The first involves charging the batteries to a fully charged level immediately after manufacturing the battery pack, then, by effective business operation, minimizing the time from manufacture to sale to an end user. Second, the designer or designers of the battery pack use low current drain designs, and integrate circuits. Typically a manufacturer will employ a combination of these two strategies to minimize the chance of delivering a reduced capacity battery pack to a customer.
However, both strategies typically result in an increased cost of manufacture. Charging battery packs prior to shipment involves equipment and personnel. Integrating subcircuits usually requires the design of custom parts. Therefore, there is a need for a means to address the problem of battery pack circuit current drain between the time of manufacture and the time of delivery, as well as during long periods of storage.